The finite electromagnetic spectrum for modern wireless communications is becoming more and more crowded. Band pass filters with high spectral selectivity are highly demanded to make sufficient use of the electromagnetic spectrum. Microstrip line emerges as a good candidate for band pass filter designs due to its advantages of low cost, planar structure and easy fabrication. Among various microstrip line band pass filters, the traditional end-coupled band pass filters using gap-couplings are simple both in structure and in design procedure. However, the applications of these kinds of filters are not so wide because the performance is too sensitive to the sizes of the feeding and coupling gaps.
On the other hand, according to the coupled resonator theory, resonator and coupling are the two key factors in microwave band-pass filter design. Past research on microwave band pass filters, especially microstrip line band pass filters, had focused on proposing various new kinds of resonators to improve filter performance or achieve special functions. For coupling mechanism, end- and edge-couplings, both of which belong to gap-coupling, dominate the coupling mechanism in microstrip line band pass filters.
Therefore, there is a need for a coupling mechanism that can at least make use of the advantages of the end-coupled structure and yet avoid the disadvantages of the gap-coupling.